The present invention relates to a multilevel phase change memory.
As is known, phase change memories use a class of materials that have the property of switching between two phases having distinct electrical characteristics, associated with two different crystallographic structures of the material: an amorphous, disorderly phase, and a crystalline or polycrystalline, orderly phase. The two phases are hence associated to resistivities of considerably different values.
Currently, the alloys of elements of group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can be used advantageously in phase change memory cells. The currently most promising chalcogenide is formed from an alloy of Ge, Sb and Te (Ge2Sb2Te5), also called GST, which is now widely used for storing information on overwritable disks and has been also proposed for mass storage.
In chalcogenides, the resistivity varies by two or more orders of magnitude when the material passes from the amorphous (more resistive) phase to the crystalline (more conductive) phase, and vice versa.